Electro-magnetic energy conductors are conventionally coated with a jacket to protect against catastrophic failures of the electro-magnetic system by short circuit. A number of materials have been used as protective jackets, such as polyvinyl chloride (PVC), polyurethane, polyimide, polytetrafluoroethylene (PTFE), expanded PTFE, fluorinated ethylene propylene, perfluoroalkoxy polymer, polyesters, silicone rubber, and nylon. These materials have been applied over the conductors in a variety of ways, including by extrusion, tape wrap, insertion within pre-formed tubes, shrink wrap, etc. As a specific example, U.S. Pat. No. 5,846,355 describes a jacket material comprising a silicone material that is imbibed into a porous polymer, such as expanded polytetrafluoroethylene, to produce a flexible and durable composite.
Various laminar structures comprising polyimide and a combination of certain fluoropolymer layers have been described for use as an insulating wire or cable wrap. For example, U.S. Pat. No. 3,616,177 discloses a construction comprising at least three layers, including a base polyimide layer, a layer of fluoroethylene propylene (FEP) copolymer and a layer of polytetrafluoroethylene (PTFE) copolymer. This patent further describes a four layer construction having FEP on both sides of the polyimide base layer, with a layer of PTFE on of the FEP layers. See column 1, lines 37-45. An alternative construction is disclosed in U.S. Pat. No. 5,399,434, which describes a tape having a polyimide core layer, inner fluorinated ethylenepropylene copolymer (FEP) layers, intermediate polytetrafluoroethylene homopolymer (PTFE) and fluorinated ethylenepropylene copolymer (FEP) blend layers and outer fluorinated ethylenepropylene copolymer (FEP) layers for use as an insulating wire or cable wrap.
U.S. Pat. No. 7,022,402 describes a film used to wrap electrically conductive materials, particularly for aerospace, high voltage machinery and/or other high performance applications. The film is an asymmetric multi-layer insulative film made by combining a layer of polyimide and a high-temperature bonding layer, the high-temperature bonding layer being derived from a high temperature base polymer made of poly(tetrafluoroethylene-co-perfluoro[alkyl vinyl ether]) (PFA) and optionally blended with from 0 to 60 weight percent poly(tetrafluoroethylene-co-hexafluoropropylene) (designated “FEP” therein). See the Abstract and the Field of Invention. A high modulus polyimide film is preferred for use as the core layer in order to provide the desired mechanical toughness, though films with a lower modulus may also be used. See column 7, lines 55-62.
U.S. Pat. No. 5,106,673 to Effenberger, et al. describes a multi-layer film stated to have improved adhesive strength and other properties. The film is made by combining a layer of polyimide and one or more layers of fluoropolymer selected from the group consisting of PTFE, thermally compatible TFE copolymers, blends thereof, PVF2, thermally compatible VF2 copolymers, blends thereof, PCTFE, thermally compatible CTFE copolymers, and blends thereof. See the Abstract. FEP is listed as a TFE copolymer in this patent (see column 5, line 27). However, it is also an object of the invention disclosed therein to reduce or eliminate the tendency of laminations made using polyimide and fluoropolymer tapes to arc-track by maximizing the PTFE content of the total fluoropolymer present (see column 3, line 3 to 7). To this end, Effenberger teaches that the adhesive layers should contain at least 40% by volume PTFE. See column 6, line 36 to 41.
U.S. Pat. No. 5,731,088 to La Court discloses a multi-layer composite comprising a polyimide film, a first layer of FEP bonded to both sides of the polyimide film, and a layer of a blend of PTFE and FEP bonded to one of the FEP layers. La Court stated that the “FEP coating provides excellent bonding of the polyimide copolymer base film layer to the subsequently applied PTFE-FEP blend layers. Without the FEP coating, the PTFE-FEP blend layers do not bond very well to the polyimide copolymer base layer unless the proportion of FEP in the blend is at least about 50%.” See column 4, lines 41-46. However, to achieve the desired arc-track resistance, La Court asserted that “the PTFE-FEP blend must contain at least 40 weight % and, preferable up to 90 weight % of the PTFE homopolymer.” See column 4, lines 56-59.
Kaneka Texas Corporation currently sells fluorocarbon coated polyimide films, such as APICAL Type AF Polyimide film designated 120AF616, which contains a polyimide base layer and 100% FEP layers bonded to both sides of the polyimide base layer. Similarly, DuPont Company currently sells a fluorocarbon coated polyimide film designated Kapton® FN, which is a general purpose polyimide film that is coated or laminated on one or both sides with Teflon® FEP fluoropolymer.
It is desirable to provide an insulation structure useful for aerospace wires or cable applications having improved heat strength and abrasion resistance.